Ink jet printers have printheads that operate a plurality of inkjets that eject liquid ink onto an image receiving surface. The ink may be stored in reservoirs located within cartridges installed in the printer. Such ink may be aqueous, oil, solvent-based, or UV curable ink or an ink emulsion. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the image receiving surface. In these solid ink printers, the solid ink may be in the form of pellets, ink sticks, granules or other shapes. The solid ink pellets or ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device that melts the ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like. In other inkjet printers, ink may be supplied in a gel form. Gel inks are also heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead.
A typical full width scan inkjet printer uses one or more printheads. Each printhead typically contains an array of individual nozzles for ejecting drops of ink across an open gap to an image receiving surface to form an image. The image receiving surface may be the surface of a continuous web of recording media, the surfaces of a series of media sheets, or the surface of an image receiving member, such as a rotating print drum or endless belt. Images printed on a rotating surface are later transferred and fixed to recording media by mechanical force in a transfix nip formed by the rotating surface and a transfix roller.
In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that eject ink from an ink filled pressure chamber and through an orifice in response to an electrical voltage signal, sometimes called a firing signal. The amplitude, frequency, or duration of the firing signals affects the amount of ink ejected in each drop. The firing signal is generated by a printhead controller in accordance with image data. An inkjet printer forms a printed image in accordance with the image data by printing a pattern of individual ink drops at particular locations on the image receiving member. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving member in accordance with image data.
In order for the printed images to correspond closely to the image data, both in terms of fidelity to the image objects and the colors represented by the image data, the printheads must be registered with reference to the imaging surface and with the other printheads in the printer. Registration of printheads is a process in which the printheads are operated to eject ink in a known pattern and then the printed image of the ejected ink is analyzed to determine the orientation of the printhead with reference to the image receiving surface and to the other printheads in the printer. Operating the printheads in a printer to eject ink in correspondence with image data presumes that the printheads are level with a width across the image receiving surface and that all of the inkjet ejectors in the printhead are operational. The presumptions regarding the orientations of the printheads, however, cannot be assumed, but must be verified. Additionally, if the conditions for proper operation of the printheads cannot be verified, the analysis of the printed image should generate data that can be used either to adjust the printheads so they better conform to the presumed conditions for printing or to compensate for the deviations of the printheads from the presumed conditions.
Analysis of printed images is performed with reference to two directions. “Process direction” refers to the direction in which the image receiving member is moving as the imaging surface passes the printhead to receive the ejected ink and “cross-process direction” refers to the direction across the width of the image receiving member. In order to analyze a printed image, a test pattern needs to be generated so determinations can be made as to whether the inkjets operated to eject the ink comprising the pattern did eject ink and whether the ejected ink landed in the correct location. The correct location refers to the positions where the ink would have landed if the printhead was oriented correctly with reference to the image receiving surface and the other printheads in the printer.
Systems and methods exist for detecting ink drops on an image receiving surface ejected by different printheads, inferring the positions and orientations of the printheads from the detected drops, and identifying correctional data useful for moving one or more of the printheads to achieve alignment acceptable for good registration in the printing system. The ink drops are ejected in a known pattern, sometimes called a test pattern, to enable one or more processors in the printing system to analyze image data of the test pattern on the ink receiving substrate for detection of the ink drops and determination of the printhead positions and orientation. In some inkjet printing systems, printheads are configured to eject a clear ink onto the ink receiving member. This clear ink is useful for adjusting gloss levels of the final printed product and for providing a protective layer over printed areas, if desired. One issue that arises from the use of clear ink, however, is the difficulty in detecting drops of clear ink ejected onto an ink receiving member with an imaging system. Because clear inks do not image well, known systems and methods for automatically aligning printheads from test patterns printed with colored ink do not enable the clear ink drops to be detected and the positions and orientations of the printheads ejecting clear ink to be inferred. Therefore, improvements to inkjet printers that enable in-situ registration of printheads that eject clear ink is desirable.